1. Field of the Invention
The present invention relates to a projection optical system, an exposure apparatus, and a device fabrication method.
2. Description of the Related Art
A projection exposure apparatus has conventionally been employed to fabricate, for example, a micropatterned semiconductor device such as a semiconductor memory or logic circuit by using photolithography. The projection exposure apparatus projects and transfers a circuit pattern formed on a reticle (mask) onto, for example, a wafer via a projection optical system.
A minimum line width (resolution) that the projection exposure apparatus can transfer is proportional to the wavelength of exposure light and is inversely proportional to the numerical aperture (NA) of the projection optical system. Along with a demand for micropatterning semiconductor devices, the wavelength of the exposure light is shortening and the NA of the projection optical system is increasing. Particularly in recent years, a projection optical system (immersion projection optical system) using an immersion exposure technique has attracted a great deal of attention as a means for increasing the NA of the projection optical system. The immersion exposure technique further increases the NA of the projection optical system by filling the space between the wafer and the final lens (final surface) of the projection optical system with a liquid.
The immersion projection optical system generally uses pure water as the liquid which fills the space between the final lens and the wafer, and uses quartz as the lens material of the final lens. From the viewpoint of design of this system, the limit of its numerical aperture is about 1.35. Under the circumstances, it is proposed to increase the numerical aperture to 1.5 or 1.65 or more by using a liquid having a refractive index higher than that of pure water and a lens material having a refractive index higher than that of quartz.
At present, LuAG (Lu3Al5O12) is attracting a great deal of attention as a lens material which transmits light having a wavelength of 193 nm and a refractive index higher than that of quartz. Since, however, LuAG is a crystal lens material, it contains crystal-structure-related birefringence. The higher the refractive index of the crystal lens material, the larger the crystal-structure-related birefringence. For example, CaF2 (calcium fluoride) has a refractive index of 1.506 with respect to light having a wavelength of 193 nm, and has a maximum crystal-structure-related birefringence of 3.4 nm/cm. On the other hand, LuAG has a refractive index of 2.14 with respect to light having a wavelength of 193 nm, and has a maximum crystal-structure-related birefringence of 30 nm/cm.
FIGS. 21A and 21B each show the crystal-structure-related birefringence distribution of an isotropic crystal lens material (flat shape). FIG. 21A shows the birefringence distribution around the <1 1 1> crystal axis (crystal orientation). FIG. 21B shows the birefringence distribution around the <1 0 0> crystal axis (crystal orientation). In FIGS. 21A and 21B, each position along the radial direction indicates the light beam propagation angle, and the radial direction indicates the propagation direction of the light beam. The length of each short line indicates the relative amount of birefringence, and the direction of the short line indicates the fast axis direction of birefringence.
Referring to FIGS. 21A and 21B, the crystal-structure-related birefringence of an isotropic crystal lens material is zero in the <1 0 0> crystal axis orientation and <1 1 1> crystal axis orientation, and is maximum in the <1 1 0> crystal axis orientation. Hence, when the <1 0 0> crystal axis and <1 1 1> crystal axis are oriented along the optical axis of the projection optical system, the light beam propagation angle increases as the numerical aperture increases, resulting in an increase in the crystal-structure-related birefringence.
To correct the crystal-structure-related birefringence, there has been proposed a technique of forming other lenses of the projection optical system using the same crystal lens materials as that of the final lens or crystal lens materials each having nearly the same birefringence as that in the final lens, and controlling the assembly angles of these crystal lens materials around the optical axis. Japanese Patent Laid-Open Nos. 2004-45692 and 2006-113533 propose other techniques of correcting the crystal-structure-related birefringence.
To correct the crystal-structure-related birefringence, the maximum angles between the optical axis and a light beam (propagating light beam) which propagates through the crystal lens materials are preferably, nearly equal to each other. U.S. Pre-Grant Publication No. 2007/0035848 proposes an optical system with such an arrangement.
International Publication WO 2006/089919 proposes a technique of correcting the crystal-structure-related birefringence not only by specifying the assembly angles of the crystal lens materials around the optical axis but also by arraying them in the light beam traveling direction in consideration of the order of the crystal lens materials.
Japanese Patent Laid-Open No. 2004-45692 discloses a technique of efficiently correcting the crystal-structure-related birefringence by orienting the <1 0 0> crystal axis of a crystal lens material, which exhibits a maximum angle between a propagating light beam and the optical axis of the projection optical system of 30° or more, along the optical axis of the projection optical system. However, Japanese Patent Laid-Open No. 2004-45692 does not take account of a high refractive index crystal lens material which has a very large crystal-structure-related birefringence (e.g., has a birefringence more than 20 nm/cm). It is difficult to correct the crystal-structure-related birefringence of such a high refractive index crystal lens material unless not only the condition of a crystal lens material in which the <1 0 0> crystal axis is oriented along the optical axis of the projection optical system but also that of a crystal lens material in which the <1 1 1> crystal axis is oriented along the optical axis of the projection optical system is defined.
Japanese Patent Laid-Open No. 2006-113533 discloses a technique of correcting the crystal-structure-related birefringence of a high refractive index crystal lens material by forming the final lens and lenses adjacent to it using MgO (magnesium oxide) and CaO (calcium oxide) having crystal-structure-related birefringences of opposite signs. However, Japanese Patent Laid-Open No. 2006-113533 does not show concrete arrangements of the crystal axes of MgO and CaO, which allow reduction in the crystal-structure-related birefringence. In practice, high-quality MgO and CaO which can be used for the exposure apparatus neither exist nor are under development.
U.S. Pre-Grant Publication No. 2007/0035848 (FIG. 2) discloses a technique of correcting the crystal-structure-related birefringence by arraying crystal lens materials such that the maximum angles between the optical axis and a light beam which propagates through these materials are nearly equal to each other. However, U.S. Pre-Grant Publication No. 2007/0035848 (FIG. 2) does not take account of the curvature radii of these crystal lens materials. It is difficult to correct the crystal-structure-related birefringence unless the curvature radii of these crystal lens materials are specified.
U.S. Pre-Grant Publication No. 2007/0035848 discloses a technique of correcting the crystal-structure-related birefringence by alternately arraying crystal lens materials in each of which the <1 0 0> crystal axis is oriented along the optical axis of the projection optical system, and crystal lens materials in each of which the <1 1 1> crystal axis is oriented along the optical axis of the projection optical system. However, U.S. Pre-Grant Publication No. 2007/0035848 does not take account of the array order of the crystal lens materials with respect to their assembly angles around the optical axis. When at least six or more crystal lens materials are arrayed, it is difficult to correct the crystal-structure-related birefringence unless the condition of the array order of the crystal lens materials with respect to their assembly angles around the optical axis is defined.